1. Field of the Invention
This invention relates generally to internal combustion engines, including insulated components exposed to combustion chambers and/or exhaust gas of diesel engines, and methods of manufacturing the same.
2. Related Art
Modern heavy duty diesel engines are being pushed towards increased efficiency under emissions and fuel economy legislation. To achieve greater efficiency, the engines must run hotter and at higher peak pressures. Thermal losses through the combustion chamber become problematic under these increased demands. Typically, about 4% to 6% of available fuel energy is lost as heat through the piston into the cooling system. One way to improve engine efficiency is to extract energy from hot combustion gases by turbo-compounding. For example, about 4% to 5% of fuel energy can be extracted from the hot exhaust gases by turbo-compounding.
Another way to improve engine efficiency includes reducing heat losses to the cooling system by insulating components of the engine, for example using insulating layers formed of ceramic materials. One option includes applying a metal bonding layer to a metal surface followed by a ceramic layer. However, the layers are discrete and the ceramic is by its nature porous. Thus, combustion gases can pass through the ceramic and start to oxidize the metal bonding layer at the ceramic/bonding layer interface, causing a weak boundary layer to form and potential failure of the coating over time. In addition, mismatches in thermal expansion coefficients between adjacent layers, and the brittle nature of ceramics, create the risk for delamination and spalling.
Another example is a thermally sprayed coating formed of yttria stabilized zirconia. This material, when used alone, can suffer destabilization through thermal effects and chemical attack in diesel combustion engines. It has also been found that thick ceramic coatings, such as those greater than 500 microns, for example 1 mm, are prone to cracking and failure. Typical aerospace coatings used for jet turbines are oftentimes not suitable because of raw material and deposition costs associated with the highly cyclical nature of the thermal stresses imposed.